The invention relates to the surface state of tires, particularly their protection against the consequences of the various antioxidants and antiozonants that they contain migrating to the surface.
It is known that certain polymers, in particular vulcanized rubber compositions based on diene polymers containing ethylenic double bonds in their main chain, are very sensitive to the action of ozone.
When an article made with such a diene elastomer composition is subjected to the action of a stress in the presence of ozone, the deleterious effect of the ozone is manifested by the appearance of surface cracks oriented perpendicularly to the stress direction. If this stress remains, or each time it occurs, the cracks grow and may cause complete failure of the article.
In order to limit this degradation, antiozone chemical compounds as well as waxes are commonly incorporated into elastomer compositions. The antiozone chemical compounds slow down the formation and propagation of the cracks under static and dynamic stressing conditions. The waxes provide additional static protection by forming a protective surface coating.
These means of combating degradation due to ozone have proved their effectiveness. However, the most effective antiozone compounds are also characterized by a very high tendency to migrate through their polymeric substrate and end up staining and coloring the adjacent surfaces. For example, yellowish or brown stains appear at the surface of the tire walls. This phenomenon is called "coloration". Surface migration of the waxes also modifies the external appearance of the surfaces of elastomer compositions, making them dull and gray. This phenomenon is called wax "efflorescence".
These migrations are particularly damaging in the case of "white-walled" tires but they may also seriously compromise the attractive appearance of "black-walled" tires whose external surfaces lose their shiny appearance and acquire a dull grayish one.
In order to preserve the attractive appearance of the white walls of tires up to the moment when they are put into service, protective coatings are usually employed. These coatings are applied to the walls after the tires have been manufactured. They are mainly based on polyvinyl alcohol ("PVA") flexibilized by plasticizers and contain "barrier" agents, for example mica particles, in order to slow down the migration of these chemical antiozonants and these waxes to the surface. U.S. Pat. No. 5,149,591 describes one of these protective coatings. However, these coatings are visible, thick, have limited mechanical strength and must be removed before the tires are put into service. The present invention proposes to solve these problems.
The mechanical behavior of uncrosslinked polymers varies as a function of temperature, from a glassy region at low temperatures, where the behavior is glass-like, that is to say rigid and brittle, to a fluid-flow region at elevated temperatures. Between these two regions is a region called the "rubbery plateau" where the behavior is rubber-like, that is to say close to that of an elastomer, as long as the molecular weight of the polymer is high enough for there to be entanglements (see: "Viscoelastic Properties of Polymers", John D. Ferry, 3rd ed., John Wiley & Sons, 1980, especially Chapters 10, 12 and 13).
The temperature at which the mechanical behavior of the polymer changes from this glass-like, rigid and brittle behavior to this rubber-like behavior is called the "glass transition temperature" (or "T.sub.g ") of the polymer. This glass transition temperature is an essential characteristic of polymers.
The glass transition temperature is usually determined by differential enthalpy analysis (see "Introduction to Thermal Analysis: techniques and applications", Michael E. Brown, Pub. Chapman and Hall, New York, 1988). This technique, more commonly known by the initials DSC (Differential Scanning Calorimetry), consists in determining the variations in specific heat of a specimen whose temperature is being raised. It demonstrates the presence of transitions or reactions which are accompanied by the release of energy (exothermic transition or reaction) or absorption of energy (endothermic transition or reaction). The glass transition is an endothermic transition.